The regulation of sleep has a homeostatic component, as evidenced by reliable and robust increases in sleep amount and sleep depth that are a consequence of sleep deprivation. While progress has been made in identifying neurochemical and gene expression changes that accompany sleep loss, identification of neuronal circuits most crucial to homeostatic aspects of sleep regulation is lacking. In the previous funding period, we examined sleep-regulatory functions of neurons in the median preoptic nuclues (MnPN) and ventrolateral preoptic area (VLPO). We demonstrated that, similar to VLPO, the MnPN contains significant numbers of neurons that are activated during both nonREM and REM sleep compared to waking, that MnPN and VLPO sleep-active neurons are GABAergic, and that the MnPN projects to several brain regions implicated in the control of arousal, nonREM sleep and REM sleep. We also demonstrated that MnPN neurons are maximally activated during sleep deprivation and that VLPO neurons are maximally activated during recovery sleep following sleep deprivation. Activation of preoptic area neurons occurs in response to total sleep deprivation as well as in response to selective REM sleep deprivation. These latter findings identify MnPN and VLPO neurons as possible components of neural circuits regulating nonREM and REM sleep homeostasis. Proposed work will answer the following critical questions: 1) Do individual MnPN and VLPO neurons become progressively activated in association with accumulating sleep need during sleep deprivation, and de-activated as that need dissipates during recovery sleep? 2) Are adenosinergic mechanisms responsible for changes in the activity of MnPN and VLPO neurons that occur during sleep deprivation and recovery sleep? 3) Do MnPN and VLPO neurons activated during total sleep deprivation and those activated during selective REM sleep deprivation project to different brain regions? Impaired ability to initiate and maintain sleep is a prominent feature in several psychiatric disorders including, depression, post-traumatic stress disorder and anxiety disorders. Dysregulation of both nonREM and REM sleep control are seen in these diseases. Proposed work will attempt to characterize the functional neuroanatomy and the neurochemical modulation of forebrain and brainstem neural circuits that underlie normal and disordered homeostatic sleep regulation.